1. Field of the Invention
The present invention relates generally to a bolometer type infrared detector having a thermal-separation structure. More particularly, the invention relates to a bolometer type infrared detector with high performance which can restrict temperature drift and noise.
2. Description of the Related Art
An infrared detector is generally classified into a quantum type infrared detector utilizing band structure of semiconductor or the like and a thermal type infrared detector utilizing variation of physical property value (resistance, dielectric constant and so forth) depending upon heat. The former has high sensitivity but requires cooling in the light of operation principle. In contrast to this, the latter is called as uncooled type because it does not require cooling. Therefore, it is advantageous in view point of production cost, maintenance cost and in comparison with the quantum type one. As the result, the thermal type infrared detector is becoming main stream of the infrared detector.
A thermal type infrared detector includes bolometer type, pyroelectric type and thermocouple type infrared detectors. In order to enhance sensitivity of the infrared detectors of any type, a thermal isolation structure, namely a diaphragm structure, is employed. Amongst, the bolometer type infrared detector has relatively superior characteristics. Particularly, the infrared detector, in which vanadium oxide (VO2) is used as bolometer material, as set forth in SPIE (1996, Vol. 2746, pp.23-31), the bolometers are arranged two-dimensionally as infrared imaging elements. Here, with taking one pixel for example, typical structure of the bolometer type infrared detector will be discussed. FIG. 3 is a plan view showing a structure of the conventional bolometer type infrared detector, and FIG. 4 is a section taken along line B-B′ in FIG. 3.
As shown in FIGS. 3 and 4, the bolometer type infrared detector is generally constituted of a thermoelectric converting portion 1 and a beam 2 supporting the same. The thermoelectric converting portion 1 is a diaphragm structure supported by the beam 2. The diaphragm structure is constituted of a supporting film 3, a bolometer thin film 4 as thermoelectric material, an electrode 6 for detecting variation of resistance of the bolometer thin film 4 and a protection film 5. These components are thermally separated from a substrate 9 by means of a air gap 8.
The diaphragm structure is formed by depositing a sacrificing layer on the substrate 9, depositing the support film 3 and the bolometer thin film 4, performing patterning the bolometer thin film 4, forming the electrode 6, depositing the protection film 5, performing patterning, such as by dry etching or the like for shaping respective layers into desired shapes and exposing the sacrificing layer around the thermoelectric converting portion 1, and finally removing the sacrificing layer from the portion around the thermoelectric converting portion 1 by etching. The air gap 8 as a portion where the sacrificing layer is removed, is a completely hollow space. The thermoelectric converting portion 1 is suspended by the beam 2. While not particularly illustrated, the tip end of the beam 2 is grounded to the substrate 9. Opposite two edges of the bolometer thin film 4 are contacted with the electrodes 6. These electrodes 6 are connected to a signal processing circuit via the beam 2. A temperature of diaphragm is elevated by absorbing infrared rays to cause variation of resistance of the bolometer thin film 4 to take out as an electric signal.
On the other hand, when the bolometer materials having low specific resistance, such as titanium (Ti), yittrium-barium copper oxide (BCO) and so forth are employed, while it is advantageous for capability of integration of the electrode and the bolometer material, it becomes necessary to employ a meander structure on the diaphragm shown in FIG. 5 in order to control a resistance value as element to a desired value. This example has been discussed in SPIE (1993, volume 2020, pp.2-10). The meander structure has narrow width and large number of times of turning back, but is not considered to be electrically influenced by the shape for using low resistance material (for example, non-uniformity of current density at turning back positions and so forth).
The bolometer type infrared detector elevates the temperature of diaphragm by absorbing infrared rays to read out resistance variation of the bolometer caused by temperature elevation of the diaphragm as an electric signal. In practice, upon operation, a current flows through the bolometer by applying bias to cause diaphragm temperature elevation by self-heating by Joule heat. Temperature elevation by Joule heat can be a cause of temperature drift upon operation and significantly influence for the electric signal depending upon variation of resistance of the bolometer. Therefore, Joule heat dependent temperature elevation should be restricted as much as possible. Therefore, a bolometer resistance is preferred to be as high as possible, and is appropriate to be about 100 kΩ.
VO2 as a material obtained relatively high value of resistance temperature coefficient (TCR) in the extent of −2%/K, has a sheet resistance of 10 to 20 kΩ at a thickness of about 1000 Å (100 nm). In order to obtain 100 kΩ as bolometer resistance for reducing influence of temperature drift, it is considered to improve bolometer shape and reducing thickness or thinning in the extent of ⅕ to 1/10 from the current thickness or wire diameter. By this, temperature drift can be improved.
On the other hand, it has been known that as the noise of the bolometer type infrared detector, 1/f noise is dominant. An amount Sv of 1/f noise is expressed by the following equation:                               S          ⁢                                           ⁢          v                =                              KV            2                    f                                    (        1        )            wherein V is a voltage, a value K as coefficient of 1/f noise is said to depend upon volume of the bolometer material or number of carriers. Namely, greater volume or smaller specific resistance (greater number of carriers), the value K becomes smaller to achieve superior performance. However, even if the resistance is increased and temperature drift is improved for the method set forth above, noise characteristics is significantly degraded by volume effect. Finally, superior performance cannot be obtained. On the other hand, even by improving the material per se, and increasing specific resistance, degradation of noise characteristics cannot be avoided due to effect of number of carriers.
Accordingly, it has been desired to increase only resistance of the bolometer with maintaining volume thereof substantailly unchanged. As a method for realizing this, meander structure as shown in FIG. 5 is considered in the prior art. In FIG. 6, there is shown a condition where wiring is formed by electrodes 6 with forming the bolometer thin film 4 into meander structure using normal VO2. Since specific resistance is high in comparison with FIG. 5, width of the meander structure is wider and number of turning backs is smaller.
In this structure, volume of the entire bolometer is held substantially unchanged as compared with FIG. 3 and only resistance is adjusted to be in the extent of 100 kΩ. In the turning back (bent) portion, a current density becomes particularly non-uniform to reduce effective volume contributing for operation. Accordingly, concerning the value K, with respect to the value premised from the volume of the bolometer, twice or three times of fatigue is caused. On the other hand, the current concentrates to the inner portion of the curved portion to be high temperature to cause adverse effect to the operation.